<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>index</title>

    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        .container {
            width: 400px;
            height: 200px;
            background-color: #ccc;

            position: absolute;
            top: 50%;
            left: 50%;

            margin-top: -100px;
            margin-left: -200px;
        }

        .left_top {
            width: 50px;
            height: 50px;
            background-color: #fc0;

            position: absolute;
            top: 0;
            left: 0;

            border-radius: 0 0 50px 0;
        }

        .right_bottom{
            width: 50px;
            height: 50px;
            background-color: #fc0;

            position: absolute;
            right: 0;
            bottom: 0;

            border-radius: 50px 0 0 0;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="left_top"></div>
        <div class="right_bottom"></div>
    </div>
</body>
</html>